Method and apparatus for providing positional information on a disk

ABSTRACT

The present invention is a method and apparatus for positioning a read/write head in a hard disk drive. The method comprises providing a disk having a at least one side with a plurality of tracks, where each of the tracks has a servo field with servo bits. The servo bits are read to provide a position signal for positioning a read/write head. The method determines a difference in position between an initial and a subsequent position of the read/write head on a track, where the subsequent location occurs after the read/write head has moved one revolution from the initial position on the track. The initial and subsequent positions are offset laterally. The method generates a compensation signal based on the initial position, the subsequent position and the difference. The position signal and the compensation signal are combined to provide a compensated position signal for positioning the read/write head. Various embodiments are described.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a non-provisional application of aprovisional application, assigned Provisional Application Ser. No.60/232,649, and filed Sep. 14, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to disk storage systems andmore particularly, methods and apparatus for providing positionalinformation on a disk in a hard drive assembly.

2. Description of the Related Art

Disk drives are magnetic recording devices used for the storage ofinformation. The information is typically recorded on concentric trackson either surface of one or more magnetic recording disks. To facilitatethe storage and retrieval of data in an orderly manner, disks aretypically organized in blocks called sectors. These sectors are locatedon the disk by a set of unique specifiers called cylinder (or track),head (or side) and sector number. The disks are rotatably mounted to aspin motor and information is accessed by means of read/write heads thatare mounted to actuator arms which are rotated by a voice coil motor.The voice coil motor is excited with a current to rotate the actuatorand move the heads. The read/write heads must be accurately aligned withthe storage tracks on the disk to ensure proper reading and writing ofinformation.

To accurately write and read data, it is desirable to maintain the headon the center of the track. To assist in controlling the position of thehead, each sector of the disk typically contains a number of servo bitsaccurately located relative to the centerline of the track. The rawsignals produced by the servo bits are typically demodulated into aposition signal which is utilized by a servo system to determine theposition of the head relative to the track, and to move the actuator armif the head is not located on the track centerline.

Due to defects in the servo patterns, the read head does not return toits original position after one revolution, as shown in FIG. 1A. Thisresults in a gap between the original (starting) position and theposition of the read/write head after one revolution. The resultingposition signal is an anomaly, and takes the form of a spike, as shownin FIG. 1B.

Accordingly, there is a need in the technology for a method andapparatus for providing servo information on a disk in a hard driveassembly while overcoming the aforementioned problems.

BRIEF SUMMARY OF THE INVENTION

The present invention is a method and apparatus for positioning aread/write head in a hard disk drive. The method comprises providing adisk having a at least one side with a plurality of tracks, where eachof the tracks has a servo field with servo bits. The servo bits are readto provide a position signal for positioning a read/write head. Themethod determines a difference in position between an initial and asubsequent position of the read/write head on a track, where thesubsequent location occurs after the read/write head has moved onerevolution from the initial position on the track. The initial andsubsequent positions are offset laterally. The method generates acompensation signal based on the initial position, the subsequentposition and the difference. The position signal and the compensationsignal are combined to provide a compensated position signal forpositioning the read/write head. Various embodiments are described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates the starting position of a typical read head and thesubsequent position of the read head after one revolution.

FIG. 1B illustrates a spiked signal resulting from the error as shown inFIG. 1A.

FIG. 2A illustrates two embodiments of a process for providingcorrection of non-centered position signal, in accordance with theprinciples of the invention.

FIG. 2B illustrates one embodiment of a process for providing acorrected position signal.

FIG. 2C illustrates the result of applying the position signalcorrection process to the error in FIG. 1B.

FIG. 3 illustrates a hard disk drive which utilizes the methods of theinvention.

FIG. 4 illustrates the general layout of the servo field region of atrack.

FIG. 5 is a block diagram of portions of an integrated circuit readchannel in accordance with the present invention.

FIG. 6A illustrates one embodiment of a typical position signal PES usedto center a read head along the centerline of a track.

FIG. 6B illustrates one embodiment of a correction signal PES_(COR) usedto correct the position signal PES in providing a centered positionsignal.

FIG. 6C illustrates one embodiment of the resulting signal obtained whenthe position signal PES is combined with the correction signalPES_(COR).

FIG. 7A illustrates one embodiment of a typical position signal PES usedto center a read head along the centerline of a track.

FIG. 7B illustrates a second embodiment of a correction signal PES_(COR)used to correct the position signal PES in providing a centered positionsignal.

FIG. 7C illustrates one embodiment of the resulting signal obtained whenthe position signal PES is combined with the correction signalPES_(COR).

FIG. 8 is a flow chart illustrating one embodiment of an initializationprocess that may be implemented prior to the position signal correctionprocess.

FIGS. 9A and 9B are flow charts illustrating one embodiment of theposition signal correction process provided in accordance with theprinciples of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention maybe used in conjunction with a defect managementsystem, as described in U.S. patent application Ser. No. 09/952,683,entitled “Servo Defect Management Scheme in Hard Disk Drives” filed onSep. 13, 2001, which has been assigned to the assignee hereof, and whichis hereby fully incorporated by reference.

As discussed earlier, due to defects in the servo patterns, the readhead does not return to its original position after one revolution, asshown in FIG. 1A. This results in a gap G between the original(starting) position P1 and the position P2 of the read/write head afterone revolution. The resulting position signal is an anomaly, and takesthe form of a spike, as shown in FIG. 1B. The present invention providesand apparatus and methods for eliminating the anomalous signal, byproviding a correction term to the position signal used to direct theread head.

FIG. 2A illustrates two embodiments of a process for providingcorrection of non-centered position signal, in accordance with theprinciples of the invention. In one embodiment as illustrated by theline A in FIG. 2A, the corrected position signal PES directs the readhead at a position N1 sectors before the gap G, to move substantiallylinearly to the original position P1. In an alternate embodiment, asillustrated by the line B in FIG. 2A, the corrected position signal PESdirects the read head from a position N2 sectors before the gap G,passes through the mid point of the gap G, to move substantiallylinearly to a position P3 that is located after the original positionP1. In one embodiment, N2 is N1/2, and P3 is located at a position N2after the gap G.

FIG. 2B illustrates one embodiment of a process for providing acorrected PES signal. The servo reference signal r is typically combinedwith an original PES signal X₀ and the resulting signal is provided tothe voice coil motor VCM, which controls movement of the read head. Inaccordance with the principles of the invention, a signal X,representing the value of the PES correction term, is added to the servoreference signal r and the original PES signal X₀, and the resultingsignal, X₁ is provided to the VCM. By adding X to the servo referencesignal and the original PES signal X₀, the VCM will control the readhead to travel along one of the two paths described in FIG. 2A and thecorresponding text.

Referring to the drawings more particularly by reference numbers, FIG. 3shows a hard disk drive 100. The disk drive 100 includes a disk 102 thatis rotated by a spin motor 104. The spin motor 104 is mounted to a baseplate 106. Also mounted to the base plate 106 is an actuator armassembly 108. The actuator arm assembly 108 includes a number of heads110 mounted to corresponding flexure arms 112. The flexure arms 112 areattached to an actuator arm 114 that can rotate about a bearing assembly116. The assembly 108 also contains a voice coil 118 that is coupled tothe magnets 119 that are mounted to the base plate 106. Energizing thevoice coil 118 moves the heads 110 relative to the disk 102. There istypically a single head for each disk surface. The spin motor 104, voicecoil 118 and the heads 110 are coupled to a number of electroniccircuits 120 mounted to a printed circuit board 122. In the followingdiscussion, only one head 110 is referenced. The electronic circuits 120typically include a read channel circuit, a microprocessor-basedcontroller and a random access memory (RAM) device.

As shown in FIG. 4, data is typically stored within sectors of radiallyconcentric tracks located across the disk 102. A typical sector willhave an automatic gain control (AGC) field 150, a synchronization (sync)field 152, a gray code field 154 that identifies the track, anidentification (ID) field 156 that defines the sector, a servo field 158which includes a number of servo bits A, B, C, D, a data field 160 whichcontains the data and an error correction code field 162. In operation,the head 110 is moved to a track and the servo information provided inservo field 158 is read and provided to the electronic circuits 120. Theelectronic circuits 120 utilize the variation in the servo bits (A-B) or(C-D) to generate Q, a positioning signal for aligning the head 110.

FIG. 5 is a block diagram of an electronic circuit 120 of the drive. Theelectronic circuit 120 includes a preamplifier 172 which is coupled to aread/write (R/W) channel circuit 174. The R/W channel circuit 174includes a R/W Automatic Gain Control (AGC), a filter circuit 176, afullwave rectifier 178 and a peak detector 180. The electronic circuit120 further comprises a microprocessor-based servo controller 182 whichincludes an analog-to-digital converter (ADC) 184, a digital signalprocessor (DSP) 186, a burst sequencer and timing circuit 188 and amemory 190, such as a random access memory (RAM) device. The DSP 186includes a logic circuit 192, a summing circuit 194 and a control logiccircuit 198.

The electronic circuit 120 is coupled to one of the magnetic heads 110which senses the magnetic field of a magnetic disk 102. When reading theservo information located in the servo field region 10 on the disk 102,the head 110 generates a read signal that corresponds to the magneticfield of the disk 102. The read signal is first amplified by thepreamplifier 172, and then provided to the R/W channel circuit 174. TheAGC data included in the read signal is provided to the R/W AGC andfilter circuit 176. The R/W AGC circuit in circuit 176 monitors the AGCdata provided by the read signal and the read signal is then filtered bythe filter circuit located in the R/W AGC and filter circuit 176. Thefullwave rectifier 178 rectifies the read signal and provides therectified read signal to the peak detector 180. The peak detector 180detects the amplitude of the read signal. The read signal is thenprovided to the ADC 184 which provides digitized samples of the analogread signal. The digitized signal is then provided to a logic circuit192 located within the DSP 186. The logic circuit 192 generates aposition signal X_(O), based on the servo bits A, B, C and D that areread by the head 110. The position signal X_(O) is provided to thesumming circuit 194. The logic circuit 192 also generates a PEScorrection signal X, based on the servo bits A, B, C, and D.

The PES correction signal X is added to the position signal XO. A servoreference signal X is also added to XO. Based on the sum of r, XO and X,a corrected PES signal, X, is generated and provided to the controllogic circuit 198. The control logic circuit 198 calculates acompensated signal as control signal Q. The resulting control signal Qis stored in memory 190. The control signal Q is subsequently providedto the actuator arm assembly 108 to move the heads 110. Alternatively,the control signal Q can be provided directly to the actuator armassembly 108 to move the heads 110.

FIG. 6A illustrates one embodiment of a typical position signal PES usedto center a read head along the centerline of a track. As discussedearlier, the read head typically does not return to its originalposition after one revolution, as shown in FIG. 1A. As a result, acorrection signal is added to the original position signal PES tocorrect this anomaly. FIG. 6A illustrates one embodiment of a PES signalmodeled as a sawtooth waveform. As shown, the period T1 of the waveformcorresponds to the time it takes for the disk to complete onerevolution. The magnitude of the waveform corresponds to an off-trackposition of +/−5%. FIG. 6B illustrates one embodiment of a correctionsignal PESCOR used to correct the position signal PES in providing acentered position signal. The PES gap C in FIG. 2A corresponds to thepeak-to-peak value S of the waveform. By implementing the techniques ofthe invention, the corrected PES signal will result in the form as shownby line A′ corresponding to the paths A as shown in FIG. 2A. In oneembodiment, the magnitude SCOR of the correction signal PESCOR is thesame as the magnitude S of the original position signal PES. However,the period T1 of the signal S is equal to the period T3 of thecorrection signal PESCOR and T2 the interval between each correctionsignal. FIG. 6C illustrates one embodiment of the resulting positionsignal obtained using the correction signal of FIG. 6B. In oneembodiment, the magnitude SR of the resulting correction signal is equalto S/2 if the original position signal S is symmetric.

FIG. 7A illustrates a second embodiment of an uncorrected positionsignal used in providing a centered position signal. FIG. 7A illustratesone embodiment of a PES signal modeled as a sawtooth waveform. As shown,the period T1 of the waveform corresponds to the time it takes for thedisk to complete one revolution. The magnitude of the waveformcorresponds to an off-track position of +/−5%. FIG. 7B illustrates oneembodiment of a correction signal PESCOR used to correct the positionsignal PES in providing a centered position signal. In this embodiment,the correction signal is a dipulse signal having a period of T5, whereT5<T1. The PES gap G in FIG. 2A corresponds to the peak-to-peak value Sof the waveform. By implementing the techniques of the invention, thecorrected PES signal will result in the form as shown by line B′corresponding to the paths B as shown in FIG. 2A. In one embodiment, themagnitude SCOR of the correction signal PESCOR is the same as themagnitude S of the original position signal PES. However, the period T1of the signal S is equal to the period T5 of the correction signalPESCOR and T2 the interval between each correction signal. FIG. 7Cillustrates one embodiment of the resulting position signal obtainedusing the correction signal of FIG. 7B. In one embodiment, the magnitudeSR of the resulting correction signal is the same as that of theoriginal PES signal.

FIG. 8 is a flow chart illustrating one embodiment of the initializationprocess used prior to the position signal correction process of theinvention. The process of the invention utilizes variables stored in afile. Before arriving at a target cylinder, various variables areinitialized. The process proceeds as follows. Beginning from a STARTstate, the process 800 proceeds to process block 810, where the gapclosure location CL and the gap closure magnitude CM are read back froma closure defect list or file. The process 800 then determines CS, thesector number at which the closure compensation process is to begin, asshown in process block 820 in the following manner:If (CL−CN/2)≧0,then CS=(CN/2)Otherwise CS=CL−(CN/2)+NS

Where:

-   -   CL is the gap closure location (measured by sector number);    -   CN is the gap compensation value (measured by sector number);    -   NS is the number of sectors per revolution on the disk.

Thus, if the gap closure location is more than half of the gapcompensation value, then CS is initialized as half of the gapcompensation value. Otherwise, it is initialized as the differencebetween the sum of the gap closure location and the number of sectorsper revolution, and half the gap compensation value. The process 800then returns to the main process flow.

FIGS. 9A and 9B are flow charts illustrating one embodiment of theposition signal correction process provided in accordance with theprinciples of the invention. The process 900 determines PC, the PEScorrection value to use based on various criteria as described below.The process 900 proceeds from a START state to decision block 905, whereit queries if the current sector SN is greater than CS, the sectornumber at which the closure compensation process is to begin. CS hadpreviously been determined as shown in FIG. 8 and the correspondingtext. If SN is greater than CS, the process 900 proceeds to processblock 910, where it determines if the difference between SN and CS isgreater or equal to CN, the length of the gap compensation (in sectors).If so, PC, the PES compensation value is set to zero (process block915). The process 900 then returns to the main process flow. During themain process flow, the original position signal is combined with the PEScompensation value to provide the resulting compensated PES value. Thecompensated PES value is then used to position the read/write head.

If at decision block 910, (SN−CS) is determined to be less than CN, theprocess 900 proceeds to process block 920, where it queries if (SN−CS)is less than (CN/2). If so, the process 900 proceeds to process block925, where PC is determined as follows:PC=CM*(SN−CS+1)/(CN+1)

where CM is the magnitude of the gap. The process 900 then proceeds toreturn to the main process flow.

If, at decision block 920, the process 900 determines that (SN−CS) isnot less than (CN/2), the process proceeds to process block 931, wherePC is determined as follows:PC=CM*(SN−CS−CN)/(CN+1)

The process 900 then returns to the main process flow.

If, at decision block 905, the process determines that the currentsector SN is not greater than CS, the process proceeds to decision block935, where it determines if (SN−CS+NS) is greater than or equal to CN.If so, the process 900 proceeds to process block 940, where PC is set tozero. The process 900 then returns to the main process flow.

Otherwise, the process 900 proceeds to decision block 945 where itdetermines if (SN−CS+NS) is less than (CN/2). If so, PC is determined asfollows (process block 950):

PC=CM*(SN−CS+NS+1)/(CN+1). The process 900 then returns to the mainprocess flow.

Otherwise, the process 900 determines PC as follows (process block 955):PC=CM*(SN−CS+NS−CN)/(CN+1).

The process 900 then returns to the main process flow.

1. A method for positioning a read/write head in a hard disk drive, comprising: a) providing a disk having at least one side with a plurality of tracks, each of said tracks having a servo field, said servo field having servo bits; b) reading said servo bits to provide a position signal for positioning a read/write head; c) determining a magnitude of a difference in position between an initial and a subsequent position of the read/write head on a track, said subsequent location to occur after said read/write head has moved one revolution from said initial position on said track, said initial and subsequent position being offset laterally; and d) generating a compensation signal based on said initial position, said subsequent position and said difference; e) combining said position signal and said compensation signal to provide acompensated position signal for positioning said read/write head.
 2. The method as recited in claim 1, wherein c) further comprises: determining a sector for initiating gap compensation based on said initial and subsequent positions.
 3. The method as recited in claim 2, wherein c) further comprises determining if a difference between said initial position and half of a length of said gap compensation is greater or equal to zero; if so, said sector for initiating gap compensation is equal to a difference between said initial position and half of a length of said gap compensation; otherwise, said sector for initiating gap compensation is equal to a sum of said difference between said initial position and half of a length of said gap compensation, and a total number of sectors on said track.
 4. The method am recited in claim 3, wherein in d) said compensation signal is further based on a current position of said read/write head and said length of said gap compensation. 